1. Field of the Invention
The present invention generally relates to convenient chemical syntheses of stable, nearly monodisperse alloy magnetic nanoparticles and an economic chemical approach to magnetic alloy nanocrystalline thin film production on a solid surface. The coercivity of the thin film can be controlled in the range between 500 Oe and 6500 Oe. This synthesis route offers a viable approach to the production of ultra-high density recording media.
2. Description of the Related Art
The storage density of commercial magnetic recording media is increasing at a compound rate of 60% per annum. As the storage density increases the individual storage bit size decreases proportionally. To control signal to noise ratios and other recording parameters, it is advantageous to maintain a large number of ferromagnetic grains per bit. Thus, the development of new magnetic media with smaller grains, high coercivity and high magnetization is required. Furthermore, to maximize the signal to noise ratio, the grains should be well isolated from each other to prevent exchange coupling between the grains, and possess a narrow size distribution. However, this scaling approach is limited by the onset of superparamagnetic behavior when the grain size falls below some material-dependent characteristic dimension.
One approach to reduce particle dimensions but still maintain sufficiently high coercivity is to take advantage of the high magnetocrystalline anisotropy found in some metal alloy systems. It is known that an assembly of very small non-interacting magnetic particles with high anisotropy possesses high coercivity. The coercivity arises because the small particles can support only a single domain state and irreversible magnetization rotation is the only possible mechanism of flux reversal.
The CoPt or FePt binary alloys are excellent candidates for this approach because of their chemical stability, and high magnetocrystalline anisotropy (thus high coercivity) arising from the existence of ordered intermetallic phases. For these alloys, magnetocrystalline anisotropies of around 7.times.10e7 erg/cm.sup.3 have been obtained, compared to 5.times.10e6 erg/cm.sup.3 for hcp cobalt-based recording media. Typical Co based recording media today have anisotropies of order 1-2.times.10e6 erg/cm.sup.3. Since the stored magnetic energy scales with the anisotropy constant and the particle volume, KV, smaller particles of high K materials like FePt and CoPt can potentially be used in future media applications. The advantage is narrower transitions and reduced read back noise. Hcp cobalt based granular thin films doped with Pt are being used today in ultra-high density recording applications, a typical composition being CoP.sub.10 Cr.sub.22 B.sub.6. Accordingly, tremendous research efforts have been focused on synthesis and characterization of near equiatomic CoPt and FePt alloys. These alloys may also have great potential for use as magnetic bias films of magneto-resistive elements, and magnetic tips for magnetic force microscopy.
A common procedure leading to CoPt or FePt alloy materials is co-sputtering of Co (or Fe) and Pt. This procedure allows little control over particle size or size distribution. The following description discloses that stable, monodisperse magnetic alloy nanoparticles and related nanocrystalline thin film can be easily synthesized by convenient chemical procedures.
The invention includes a convenient chemical way to prepare stable monodisperse Fe/Pt alloy magnetic nanoscale materials, an approach to form smooth nanocrystalline films on a variety of substrates, and exploring the possibility of using the developed materials as ultra-high density recording media.